The following formula can be used to approximate the estimate of change: where Y1 and Y2 are the estimates for the two periods. The value of p is the correlation coefficient between Y1 and Y2. The value of p ranges between -1 and 1, with 1 being the perfect positive linear association. One can generally use the sample overlap to approximate the correlation coefficient as follows: - For the provinces: use p = 5/6 for month-to-month changes, and p = 0 for year-to-year changes.